FIGS. 1-3 illustrate a layout topology for a conventional Nwell diode 100 that is employed as an ESD diode for signal output circuitry. As shown in FIG. 1, Nwell diode 100 includes an Nwell 102 formed in a P-type substrate 104. Elongated N+ doped regions 106 and P+ doped regions 108 of equal length are formed within Nwell 102 as shown and separated by field oxide 107 as shown in FIGS. 2 and 3 (oxide 107 not shown in FIG. 1). A P+ doped guard ring structure 110 coupled via conductive (metal) pad 124 to negative supply rail (VSS) is formed as shown to surround elongated N+ doped finger regions 106 and P+ doped regions 108, with a positive supply rail (VDD) coupled via conductive (metal) pad 120 to N+ doped regions 106 and a conductive (metal) pad 122 of P+ doped regions 108 coupled to an output signal node or pad of an output buffer circuit. FIG. 2 illustrates section A-A of FIG. 1 and its parasitic vertical PNP bipolar device which is active during the occurrence of an electrostatic discharge (ESD) event, that causes a portion of the ESD current (IE) from conductive pad 122 to flow to VSS. FIG. 3 illustrates section B-B of FIG. 1 and its associated parasitic lateral PNP bipolar device which can also be active during the occurrence of the same ESD event.
During occurrence of an ESD event, a portion of IE flows vertically through vertical parasitic PNP bipolar device 121 to P-type substrate 104 and to VSS via P+ guard ring 110 as shown in FIG. 2 in a manner which is generally beneficial. At the same time, a portion of IE can also flow laterally through lateral parasitic PNP bipolar device 123 to P-type substrate 104 and to VSS via substrate 104 and P+ guard ring 110 as shown in FIG. 3, in manner that can cause early breakdown of lateral bipolar device 123 relative to vertical bipolar device 121, which usually leads to early failure at the ends of each of the P+ doped fingers 108. In this regard, the depth 332 of Nwell 102 sets the base width of the vertical parasitic PNP bipolar device 121 (as shown in cross section A-A of FIG. 2), while the lateral width 330 of the region of Nwell 102 that overlaps the ends of P+ doped fingers 108 sets the base width of the lateral PNP device (as shown in cross section B-B of FIG. 3).
FIG. 4 illustrates another type of a layout topology for a conventional Nwell diode 400 that is employed as an ESD diode for signal output circuitry. As shown in FIG. 4, Nwell diode 400 includes an Nwell 402 formed in a P-type substrate 404. Elongated P+ doped regions 408 having conductive (metal) pads 422 are enclosed by N+ doped regions 406 having conductive (metal) pads 420, each of which are formed within Nwell 402 and separated by field oxide (oxide not shown in FIG. 4). A P+ doped guard ring structure 410 coupled to negative supply rail (VSS) by conductive pads 424 is formed as shown to surround N+ doped enclosure region 406 and elongated P+ doped regions 408, and a positive supply rail (VDD) coupled via conductive pads 420 to N+ doped regions 406 and signal pads 422 of P+ doped regions 408 coupled to an output signal node or pad of an output buffer circuit.
Deep nwell (DNW) is a well known process option for semiconductor fabrication processes which is used to create distinct regions of P-type material which are electrically isolated from the underlying P-type substrate. Here N-type dopant is implanted in regions defined by DNW masking regions, such that the impurity concentration peaks at a distance below the surface of the P-type substrate. At the same time, the DNW masking regions are fully encircled by an annular ring of N+/Nwell which in turn electrically links or connects with the deep DNW dopant layer. This creates an isolated P-type region, which is junction isolated from the P-type substrate on the sides by N+/Nwell and on the bottom by DNW. Similar geometrical patterns of equal length elongated P+ doped regions and N+ doped regions, or N+ doped regions enclosed by P+ doped regions have been employed for forming DNW ESD diodes for signal output circuitry. Here, a complimentary parasitic vertical NPN is formed between the pad connected N+ regions, the isolated P-type region and the underlying DNW region and a parasitic lateral NPN formed between the edges of the pad connected N+ regions, the isolated P-type region and the annular N+/Nwell ring which encircles the DNW region.